1. Field of the Invention
The present invention generally relates to an illumination system, and more particularly, to an illumination system used for a projection apparatus.
2. Description of Related Art
Referring to FIG. 1, it is a diagram of a conventional illumination system 100 for a projection apparatus. The conventional illumination system 100 includes a red light emitting diode (red LED) 110R, a green light emitting diode (green LED) 110G, a blue light emitting diode (blue LED) 110B, two dichroic mirrors 120a and 120b, a lens array 130 and three collimators 140. The red LED 110R, the green LED 110G and the blue LED 110B respectively emit a red light beam 112R, a green light beam 112G and a blue light beam 112B. The collimators 140 are respectively disposed in fronts of the light-emitting surfaces of the red LED 110R, the green LED 110G and the blue LED 110B so as to converge the light into the red light beam 112R, the green light beam 112G and the blue light beam 112B.
The dichroic mirror 120a is disposed on the transmission paths of the blue light beam 112B and the red light beam 112R, and the dichroic mirror 120a passes through the blue light beam 112B and reflects the red light beam 112R. The dichroic mirror 120b is disposed between the dichroic mirror 120a and the lens array 130 and located on the transmission paths of the blue light beam 112B, the red light beam 112R and the green light beam 112G. The dichroic mirror 120b passes through the blue light beam 112B and the red light beam 112R and reflects the green light beam 112G. In addition, the lens array 130 is for converging the red light beam 112R, the green light beam 112G and the blue light beam 112B.
Usually, a projection apparatus employing only one light valve sequentially projects a red image, a blue image and a green image onto a screen, and the three color images are superposed into a full-color image on the screen, therefore, the red LED 110R, the green LED 110G and the blue LED 110B are alternately emitted. However, such a scheme would make the conventional illumination system 100 providing a weaker light beam, which leads to a lower image luminance on the screen.
The following table 1 gives out the image luminance generated by the red LED 110R, the green LED 110G and the blue LED 110B on the screen and the coordinate points (x, y) corresponding to the red light, the green light, the blue light and the white light on a CIE1931 chromaticity diagram, which stands for the chromaticity diagram created by the International Commission on Illumination in 1931.
TABLE 1image color andluminous power of the LEDluminance on the screenred lightgreen lightblue lightcolorxyluminance100%0%0%red0.7070.291380%100%0%green0.3010.6922110%0%100%blue0.1400.04021100%100%100%white0.2670.281270
Referring to FIG. 2, in general speaking, a natural light source is not a light source with a single wavelength. For example, when a natural light is red one, the light also contains other color light, such as green light and blue light, although the other color light occupies a weaker component than the dominant red light. Accordingly, the coordinate points corresponding to a natural light are located within a triangle 50 formed by three connection points of coordinate points R1, G1 and B1 on the CIE1931 chromaticity diagram. However, the conventional illumination system 100 employs LED light sources to provide red light, green light and blue light, wherein the LED light sources are single wavelength light sources which are corresponding to the coordinate points R2, G2 and B2 on the CIE1931 chromaticity diagram, respectively. The coordinate points R2, G2 and B2 are beyond the triangle 50, and the three connection lines thereof form a triangle 60 with a greater area than the area of the triangle 50. Thus, images on the screen by using the conventional illumination system 100 has excessive color saturation, which makes the image look less natural.